This invention relates to electronic article surveillance systems and, in particular, to tags for use in such systems.
One form of tag employed in present electronic article surveillance systems utilizes a circuit which is arranged to receive one or more signals at one or more preselected frequencies and, in response thereto, reradiate a desired or predetermined tag signal at a frequency related to the received one or more frequencies. In some systems of this type, the received signal is at a single high frequency and the predetermined tag signal which is reradiated is at a harmonic of that frequency. In other systems, two high frequency signals are received and the reradiated tag signal includes a signal whose frequency is at the sum of the two received frequencies. In yet other types of systems, one received signal is at a high frequency and another received signal is at a low frequency and the reradiated tag signal comprises a signal at the higher frequency modulated by a signal at the lower frequency. In these types of systems, the tag circuit usually includes a non-linear element such as, for example, a diode, for establishing the reradiated tag signal.
When using the above-described tags in an electronic article surveillance system, a transmitter transmits the signals at the one or more preselected frequencies into a surveillance zone. When a tag passes through the surveillance zone, the tag receives the signals and develops the reradiated predetermined tag signal. A receiver of the system is tuned to a predetermined frequency which depends upon the character of the reradiated tag signal (i.e., whether it is a harmonic of the received signal, or at the sum frequency of the received signals or a modulation of the received signals). The receiver, upon detection of the reradiated tag signal, then activates various alarms, or generates other appropriate signals, to indicate the presence of the tag and, therefore, the article in the zone.
Since detection of the tag is based upon the receiver detecting the reradiated predetermined tag signal, changing the tag circuit to prevent reradiation of this signal effectively deactivates the tag. In prior tags of the present type, a variety of techniques for accomplishing this have been used.
In U.S. Pat. No. 4,063,229, issued on Dec. 13, 1977, to John Welsh and Richard N. Vaughn for "Article Surveillance", and assigned to the same assignee hereof, the disclosed tag is deactivated by altering the semiconductor diode used to establish the reradiated tag signal. In this case, to deactivate the tag, the semiconductor diode is burnt out by a relatively high power RF field which is inductively coupled to the tag. In U.S. Pat. No. 4,021,705, issued May, 3, 1977, to George Jay Lichtblau for "Resonant Tag Circuits Having One or More Fusible Links', there is described a tag whose tag circuit is altered via one or more fusible links to deactivate the tag. Each fusible link is able to be fused by a radiated high energy RF field of a predetermined frequency. The fusing of a fusible link changes the value of the inductors of the tag circuit, thereby changing its resonant frequency from that of the transmitted signal, whereby the tag is deactivated.
Both of the aforesaid deactivation techniques require the use of a high energy RF field which may not be desirable in many surveillance system applications. In U.S. Pat. No. 4,318,090, issued Mar. 2, 1982, to Douglas A. Narlow and Eugene Stevens for "Apparatus For Deactivating A Surveillance Tag", and also assigned to the same assignee hereof, there is described a wand like probe which can be placed in contact with terminals of a tag to deactivate the tag. The wand applies a low energy current through the diode of the tag circuit, thereby destroying the unidirectional characteristics of the diode and preventing the diode from establishing a reradiated tag signal. While the wand alleviates the need to use a high energy RF field, the wand cannot be used to remotely deactivate the tag.
A further limitation of the above described deactivatable tags is that they are not capable of being restored to an active state after being deactivated. Therefore, a tag, upon deactivation, may not be used again.
It is, therefore, a primary object of the present invention to provide an improved tag of the above-described character.
It is further object of the present invention to provide a tag that can be remotely deactivated by a low energy field.